Glycolaldehyde (also known as hydroxyacetaldehyde) is a well-known and desirable compound that is a useful intermediate for the preparation of other valuable products. There are applications for the pure compound, its aqueous solutions, and solutions containing mixtures of glycolaldehyde and other low molecular weight carbonyl compounds.
For example, glycolaldehyde may be applied as a cross-linking agent for proteinaceous materials (K. F. Geoghegan et al., “Reversible reductive alkylation of amino groups in proteins”, Biochem. Vol. 18, 5392–5399, 1979) and other amino containing materials. One advantageous application of glycolaldehyde in this manner is its use in the strengthening of sausage casings.
One particular class of application where glycolaldehyde or its mixtures with other small aldehydes are especially useful is as browning promoters for food colouring and flavouring applications. Various types of browning reactions are known to occur in the cooking of foods. The most important type is the amino-carbonyl reaction (also known as the Maillard Reaction) in which aldehydes, ketones and reducing sugars react with amines, amino acids and proteins.
It has long been known that glycolaldehyde and certain other small carbonyl compounds have very desirable properties as browning agents for foods. For example, in a review in 1953 (“Browning Reaction Theories Integrated in Review. Dehydrated Foods. Chemistry, Vol. 1, Oct. 14, 1953), J. E. Hodge pointed out that glycolaldehyde, glyceraldehyde, pyruvaldehyde, dihydroxyacetone, acetoin and diacetyl, which form upon thermal disintegration of sugars, are some of the most highly reactive browning compounds. Furthermore, Hodge indicated that glycolaldehyde is a product of the thermal fragmentation of sugars on heating their aqueous solutions.
In his review (A. Ruiter, “Color of Smoked Foods”, Food Technology, May, 1979, pp54–63) Ruiter discussed the importance of glycolaldehyde and other carbonyl compounds found in liquid smokes, as active browners.
Later on, Hayashi and Namiki, (“Role of Sugar Fragmentation in an Early Stage Browning of Amino-carbonyl Reaction of Sugar with Amino Acid”, Agric. Biol. Chem., Vol. 50, pp 1965–1986, 1986) made a quantitative study of the relative efficacies of various carbonyl compounds in the amino-carbonyl reaction. They defined the “browning ability” of the carbonyl compound as being inversely proportional to the time taken for the Optical Density (OD) of the reaction product mixture of the amino acid alanine and the carbonyl compound to reach a specified value. They found that among the C2 and C3 carbonyl compounds produced by fragmentation of sugars, glycolaldehyde had the highest browning ability. Thus the data of Hayashi and Namiki make it quite clear that glycolaldehyde has superior browning properties in the Maillard reaction.
In their U.S. Pat. No. 4,876,108, Underwood at al introduce a “browning index” as being proportional to the OD of the reaction product mixture of the amino acid glycine and the carbonyl compound(s), after a specified period of time. It is therefore expected that this index will be directly proportional to the “browning ability” defined by Hayashi and Namiki so that both indices will give a similar ordering of the browning power of aldehydic solutions resulting from the pyrolysis of carbohydrates.
In their comprehensive review (L. Toth and K. Potthast, “Chemical Aspects of the Smoking of Meat and Meat Products”, Advances in Food Research, Vol. 29, pp87–158, 1984, Toth and Potthast summarize earlier and extensive works which show clear correlations between a smoky flavour and the phenolic content of typical “liquid smoke” products. Fujimaki et al (“Analysis and comparison of flavor constituents in aqueous smoke condensates from various woods”, Agricultural and Biological Chem., Vol. 38, p45, 1974) concluded that apart from phenols, smoke flavor is due mainly to carbonyls and lactones with higher boiling points.
In summary, the prior art establishes that low molecular weight carbonyl compounds, especially glycolaldehyde, produced by the decomposition of sugars and other carbohydrates are excellent browning agents while the smoky flavor of typical liquid smoke products is due principally to phenols.
Not all fission products of sugar are effective browning agents. For example, formaldehyde is not only inactive, but has been said to be an inhibitor of the amino-carbonyl and other types of browning reactions (Ruiter, 1979).
As formaldehyde is well known to be a hazardous substance, it would be desirable to minimize the amount of formaldehyde by-product and to maximize the ratio of glycolaldehyde to formaldehyde in a glycolaldehyde mixture or aqueous solution. Minimizing formaldehyde production is of particular importance where the glycolaldehyde solution is intended as a food browning agent.
With regard to the production of glycolaldehyde, its preparation from dihydroxymaleic acid is well known. However, this method is ill suited to large-scale production of glycolaldehyde and is impractical for industrial usage. Accordingly, several alternative methods and processes for production of glycolaldehyde are found in the prior art.
Chan (U.S. Pat. No. 4,477,685) and Auvil (U.S. Pat. No. 4,503,260) have disclosed processes for catalytic synthesis of glycolaldehyde from mixtures of carbon monoxide and hydrogen. However, these processes require high pressure and expensive catalysts. There have been several methods disclosed for the vapour phase catalytic conversion of ethylene glycol to glycolaldehyde, e.g. Seto et al, Japan Official Patent Gazette, Dec. 10, 1991, 91279342. However, such methods suffer drawbacks such as low conversion, low yields of glycolaldehyde, and formation of high concentrations of byproducts.
An alternative approach to glycolaldehyde production is through the use of thermochemical methods. It has long been known that glycolaldehyde is a product of the pyrolysis of ligno-cellulosic materials; however, yields were typically very small. Scott et al. discovered that relatively large yields of glycolaldehyde could be obtained from cellulosic feedstocks under so-called fast pyrolysis conditions (J. Piskorz, D. Radlein and D. S. Scott, “On the mechanism of the rapid pyrolysis of cellulose”, J. Anal. Appl. Pyrol., Vol. 9, 121–137, 1986). The observations of Scott et al. subsequently led to the patenting of a fast pyrolysis process for producing liquid smoke rich in glycolaldehyde (Underwood and Graham, U.S. Pat. Nos. 4,876,108 and 4,994,297).
Scott has also described the production of glycolaldehyde by the pyrolysis of starch (“Production of Hydrocarbons from Biomass using the Waterloo Fast Pyrolysis Process”, DSS Contract 23283-8-6067, Renewable Energy Div., Energy Mines and Resources Canada, Ottawa, Canada, January 1991). The maximum yiled of glycolaldehyde was only 15%.
Pyrolysis of monosaccharides and oligosaccharides including sugars is also well known to produce glycolaldehyde among other carbonyl compounds. For example, Fagerson (“Thermal Degradation of Carbohydrates, A Review”, J. Agric. Food Chem., Vol. 17, 747–750, 1969) reported the presence of glycolaldehyde in the pyrolysates of glucose and other carbohydrates in dry systems. It is pointed out that the major volatile products from the pyrolysis of starch, cellulose, sucrose, maltose and glucose were essentially identical.
Kang et al (“Ketene formation from carbohydrates”, in “Thermal uses and properties of carbohydrates”, F. Shafizadeh, K. V. Sarkanen and D. A. Tillman (eds.), Academic Press, N.Y., 1976) obtained up to 16% of ketene from pyrolysis of glucose, among other sugars, at an optimal temperature of 700° C. Since they also found that ketene could form semi-quantitatively from glycolaldehyde, they concluded that ketene probably formed by a mechanism involving the first formation of glycolaldehyde from glucose. The yield of ketene from fructose was only 8% under the same conditions and that from sorbitol only 0.9%. The implication of this work by Kang et al is that some sugars are likely to give enhanced yields of glycolaldehyde. Lowary and Richards, (Carbohyd. Res., Vol. 218, pp157–166, 1991) have also reported a yield of 12.8% by weight of glycolaldehyde by the vacuum pyrolysis of the cyclodextrin cycloheptaamylose, as a starch model compound, catalysed by trace amounts of sodium chloride.
With respect to the hydrous thermolysis of sugars, their fragmentation in aqueous solutions was first reported by Nodzu (R. Nodzu, Bull Chem. Soc. Japan, Vol. 10, 122–130, 1935). However, although there were indications of small amounts of glycolaldehyde, the principal carbonyl compounds identified were acetol, pyruvaldehyde, diacetyl and pyruvic acid. Sattler and Zerban (“Volatile Decomposition Products of Sugars in Aqueous Solutions”, J. Am. Chem. Soc. Vol. 70, 1975, 1948) also pointed out that distillation of sugars from 5% sodium bicarbonate solutions gave a “relatively large” amount of acetol among the volatile products but little glycolaldehyde.
It was noted by Scott (“Chemicals and Fuels from Biomass Flash Pyrolysis”, DSS Contract 38ST 23216-6-6164, Renewable Energy Branch, Energy Mines and Resources Canada, Ottawa, Canada, February 1988) that addition of water to the pyrolysis liquid caused separation of a heavy tar phase with enrichment of glycolaldehyde in the supernatant aqueous phase. Scott also taught the removal of colour and phenolic materials from the aqueous phase by extraction with water-insoluble solvents like methylene chloride and by absorption on ion exchange resins.
Stradal et al. subsequently patented processes for the production of “precipitated glycolaldehyde” by pyrolysis of carbohydrate containing feedstocks (U.S. Pat. No. 5,252,188, “Process for producing hydroxyacetaldehyde”, issued Oct. 12, 1993) and the use of said “precipitated glycolaldehyde” for food browning applications (U.S. Pat. No. 5,393,542, “Process for producing hydroxyacetaldehyden”, issued Feb. 28, 1995). The precipitated glycolaldehyde was produced from the pyrolysis liquid product by water-separation of the pyrolysis liquid and subjecting the obtained aqueous extract to a combination of multiple distillation steps followed by a solvent-induced precipitation.
In U.S. Pat. No. 5,292,541, issued Mar. 8, 1994 and U.S. Pat. No. 5,397,582, issued Mar. 14, 1995, Underwood et al. disclose a method for producing a browning liquid product by pyrolyzing sugars and starches. The pyrolysis process involves heating the sugars or starches at rates of greater than 1000° C. per second under conditions in which the vapour residence time is preferably less than 0.6 seconds and quenching of the volatile product to less than 300° C. in less than 0.6 seconds.
Following the method of Underwood et al., sugars or starches are pyrolyzed, the unpyrolyzed material and solid byproducts are separated from the vaporous products of pyrolysis which are then condensed or, alternatively, quenched with water or the cooled re-circulated product itself to produce a water soluble product. The water-soluble product is then purified by concentration and extraction with a water-insoluble organic solvent. Optionally, the water-soluble product may be further contacted with an ion-exchange resin and finally diluted with water to produce the desired product.
The multiple extraction and purification steps required demonstrate that this method produces a significant amount of undesired by-products along with the glycolaldehyde, which must be removed at additional expense by the stated methods. The final products are solutions relatively rich in glycolaldehyde marketed under the trade name Maillose™.
Unsurprisingly in view of the prior art, the product is claimed to have good browning properties but weak smoke flavor.
Although Underwood et al. have mentioned the use of solutions of sugars as feedstocks in U.S. Pat. No. 5,397,582, issued Mar. 14, 1995, it is clear from their examples that they have not found or recognized the great benefits that can be realized by pyrolyzing dilute solutions of aldose sugars according to the method disclosed herein. They describe the pyrolysis of dextrose (glucose) powder to give a glycolaldehyde yield of only 21.3% by weight of the glucose.
Underwood et al. also present the results of tests designed to elicit the tendency of various sugars to yield glycolaldehyde on pyrolysis. These tests involved the pyrolysis at 250° C. within a gas chromatograph of 5% aqueous solutions of sugars. The results suggest that glucose and sucrose in particular are among the worst possible candidates, giving only 900 and <10 ppm respectively of glycolaldehyde.
In the '541 patent, Underwood et al. assert that the “theoretical yield” of glycolaldehyde from pyrolysis of sugars is 38%. Surprisingly, we have now found that pyrolysis of aqueous solutions of some sugars, especially glucose, under certain conditions result in glycolaldehyde yields up to 70% by weight of the sugar. The surprising yields from the present inventive method are nearly double this supposed limit and therefore suggest the assumptions on which their “theoretical yield” are based are incorrect.
Overall, there is an ongoing need for an improved method of producing glycolaldehyde-rich solutions from inexpensive feedstocks, with as large a yield as possible and with minimum amounts of other compounds, especially formaldehyde. There are also advantages to a method that allows production of glycolaldehyde-rich solutions that can be used for food-browning and other applications with only minimal treatment.